Émail brun (from French: “brown enamel”) was developed in the Rhineland area around the time that they adopted the champlevé technique. Contrary to the name, it is not an enamel technique but rather a lacquering method like niello. A layer of linseed oil was applied to the surface of a copper base which turned a deep reddish brown on heating. Usually the brown layer was then engraved and the engraving was then gilded. The thicker the applied linseed layer, the darker the final color would be.
This is a surprisingly durable coating as can be seen from the enameled plaque on the tomb of Geoffrey de Plantagenêt in the cathedral of Le Mans, France which dates from the second half of the 12th century.